Electrical surveying in drill holes



Nov. 26, 1940. R. D. ELLIOTT ELECTRICAL SURVEYING IN DRILL HOLES 3 Sheets-Sheet l Filed May 17, 1957 NOV. 26, 1940. R D. ELLIOTT 2,222,608-

ELECTRICAL SURVEYING IN DRILL HOLES Fil ed May 17, 1957 3 Sheets-Sheet 2 z i WW Nov. 26, 1940. R. D, ELLIO TT 2,222,608

ELECTRICAL SURVEYING IN DRILL HOLES Filed May 17, 1937 3 Sheets-Sheet 3 Patented Nov. 26, 1940 UNITED STATES PATENT crime I I suffix m am. notes I trier:-

Surveying Corporation, Houston, Tea, a. corporation of Delaware a Application May 17 1937, Serial No. 143,092 29 Claims. (01. 115-182) This invention relates to the art of earth exploration by electrical methods, and particularly to the exploration of formations penetrated by drillholes.

55 rect current source. and using indicating instruments responsive to alternating current. However, when alternating current is employed with the apparatus of the -Huber patent, :a gradual drift of the recorded graphs may occur due to the cable of the type dlsclosedin Flgs.,2 and 3.

- In Huber United States Patent No. 2,072,950 fact that the impedance characteristics of the. 5

issued March 9, 1937, on Method of and appa-' cable conductors connecting the electrodes in the ratus for electrically exploring earth formations, hole and ,the surface equipment may change as a method of electrically exploring the formation the cable is wound on to and unwound from surrounding a drill hole is described, which the winch which controls it. This gradual drift method consists. (1) in traversing the drill hole of the curves does not reduce their value in in 10 with a pair of vertically'spaced electrodes on dicating the nature of the formation traversed, the end of a cable, each electrode being conbut is undesirable in that-the curves may drift nected to the surface by an independent lnsubeyond the recording limits of the record aplated conductor in the cable; (2) applyi g potenparatus and the record strips employed.

5 tial from an external source between ground and i l broad object of the present invention is to o f aid cable conductors at the surface, and eliminate .the drift referred to when practicing measuring and recording variations in resistance the, methods of the Huber patent with alternatof the circuit'as the electrodes are moved through s current f k the hole on the cable;.and (3) measuring and re- Another object is to increase the sensitivity of cording the yariatlons in potential between the apparatus of th Hu er patent when em- 20 ground and the other cable conductorat the p y'ed w alternating current surface as the electrodes are moved through the, Additi na more sp fi o t r with hole, these variations in potential resulting" largevarious features of the invention, will become. a 'ly, ifnot substantially entirely, from the elecpar nt from. the detailed descrip f h trlc field created in-the formation adjacent the vention which follows. 25 other electrode by the potential impressed on the drawings: first. mentioned electrode. I Fig. 1 is a schematic diagram of a system for The two sets of measurements are-recorded in drill hole exp hen employin 8 two [the form of .two graphs. which have been found conductor cablevery useful in log in oil wells. ,The graph .pro- Fig. 2 iso. schematic diagram indicating a vari- 30 duced' by operation No. 2 indicates the resistance atlon f h t m Shown n F mp y 8 change in'the formation immediately adjacent cab e co -fo s pa at ly i sul te and the drill hole and the graph produced by'opera- Shielded conductors,

, tion No. 3 indicates resistance changes in the Fig. 3 is a detailed cross sectional View. lllus- 5 formation at a distance from the hole appr itratinglthe construction of a cable that 'maybe mately equal to the spacing between the elecm y d in the system sh w i Fl 2.- I v trodes. Fig. 4 is a schematic diagram, illustrating still The Huber patent discloses a-system employing another modification of the system, shown in direct current, which has been found satisfactory" Fl employ a cable vi thre s p ely 40 under most conditions. However, it has long been insulated conductors and an external metallic 40 known that for earth explorations under some shield over a portion of the lower end of the cable. conditions alternating current is superior to di- P18- 5 is a cross section indicatin a possible re'ct current. Thus withalternating' current pocable nstru ti hat y be mplo ed in the larizatlon errors are minimized, and, further-u. system shown in Fig. 4.-

more, the effects of extraneous earth currents; Fi 6 i as miii diagram indicating 45 (which are usually unidirectional), can'be largemodification of the system shown in Fig.2. v 'ly. eliminated by employing indicating equipment Fig. 7 is an elevation view of a complete.elec-'- incapable of responding to the extraneous earth trode structure that-may be employed in the syscurrents butadapted to respond to alternating tem shown in'Fig. 1. currents of the particular frequency employed. Figs. 8 and 9 are enlarged vertical sectional vThe apparatus and methods disclosed in the views of the electrode structure shown in Fig. 7. Huber patent can be employed 'wtih alternating Fig. 10 is a detail vertical sectional view showcurrent'ias well as with direct current by substiing an electrode structure similar to that distuting an alternating current source for the diclosed in Figs. 7 to 9; but modified for use with 9.

Referring first to the schematic diagram of Fig. l, the apparatus there represented comprises a cable I adapted to be raised and lowered in a drill hole 2 penetrating a formation 3 in which an electrical survey is to be made. The bore hole may have a short length of casing 4 at the surface end but where the electrical characteristics of the formation surrounding a drill hole are to be determined the major portion of the hole should be uncased.

The cable I may contain a pair of insulated conductors 5 and 6 connected at their lower ends to vertical spaced apart electrodes 1 and 3, respectively. The upper end of the cable is wound on a winch 9 which is used to raise and lower the cable in the drill hole and the upper ends of the conductors 5 and 6 may be permanently connected to slip rings I0 and II on the winch which make sliding contact with brushes I2 and I3, respectively, these brushes functioning to complete connection between the upper ends of the cable conductors and the electrical apparatus at the surface. The cable I is wrapped around a pulley I5 so that the pulley rotates in proportion to the vertical movement of the cable. Pulley I5 rotates the shaft I6 which drives a shaft I! at reduced speed through a speed reducing mechanism I8 of any desired type. The

shaft I'I functions to rotate a reel I3 for winding up a photographic film on which electrical measurements are recorded by recording apparatus to be described. The shaft I] is coupled to a. shaft 20 which drives a rotatably mounted nut 2| through bevel gears 22 and 23. The nut 2| may be suitably supported for rotation in bearings 24. Rotation of the nut 2| functions to shift a shaft v25 longitudinally for a purpose to be described later. The brush 12, connected through the cable conductor 6 with the electrode 8, is connected through a reactance element 30 to one diagonal point 3I of a Wheatstone bridge 32 comprising four impedance elements 33, 34, 35 and 36, respectively, the impedance element 35 having a variable tap 31 thereon which constitutes the diagonal of the bridge opposite the diagonal 3I. One of the other diagonal points 33 and its opposite diagonal point 39 are connected to a source of alternating current 40 which is employed to energize the bridge. The diagonal point 38 is also connected to ground by a ground conductor 4I. This ground connector may, if

desired, be made to any suitable structure, such as the well casing 4. In operation the impedance elements 33, 34 and. 35 constitute the known or fixed resistance arms of the bridge and the circuit including the ground conductor 4I, the

formation 3, the electrode 8, cable conductor 5,

the slip ring I0, brush I2, and the reactance element 30 in series constitute the unknown arm of the bridge. Adjustments of the bridge to a desired condition of balance or unbalance may be had by moving the tap 31, and/or, if desired, one or more of the impedance elements 33, 34, 35 may be made variable. The diagonal points 3| and, 31 constitute the output diagonals of the bridge and they are connected directly to the primary winding 42 of a transformer 43. If the bridge is adjusted to approximate balance then any slight variations in the impedance of the circuit including the electrode 3- and the formation 3 produces a substantial percentage variation in potential across the output diagonal points 3'I and 31, which potential is applied to the primary winding 42 of transformer '43." Die transformer 43 is also provided with a secondary winding 44 which is connected through a bridge type rectifier 45 and a variable resistance 45 to a galvanometer element 41 which is positioned to reflect light from a lamp 48 onto the photographic film 49. 7

The current applied between the ground connection (at the ground conductor H) and the electrode 8 produces anelectric field in the formation and variations in this field produce varying potentials in the other electrode 1. These potential variations are applied throughthe cable conductor '5, the slip ring II to the'brush I3, and thence through the secondary winding 50 of a transformer 5| to one input terminal 52 of an amplifier 53. The other input terminal 54 of the amplifier is connected to ground through a conductor 55. The varying potentials produced in electrode 1, together with additional potentials that may be added thereto by means of the transf former 5I, are amplified in the amplifier 53 and applied to the input terminals of the bridge type rectifier 56. The output terminals of this rectifier 56 are connected to a second mirror gab vanometer element 5'1 through a variable resist ance 58 and a resistor 59 in series.

Except for the introduction of the reactance element 30, the A. C. source 40, the transformer 43, rectifier 45, the transformer 5I, the amplifier 53, the rectifier 56 and the resistor 53, the structure so far described with reference to Fig. 1 is similar to the circuit disclosed in the Huber patent referred to, and would function in the same general manner if a D. C. source were substituted for the A. C. current source 40. Thus as the cable I is lowered or raised to move the electrodes I and 3 through the hole the changes in resistance of the formation closely adjacent the electrode 8 in the primary circuit would apply varying potentials to the mirror galvanometer 41, causing the latter to trace a curve 60 on the moving film 43 and the variations in the electric field adjacent the electrode 1 would cause the mirror galvanometer to trace a second curve 6| on the moving film 49.

When employing direct current, the movements of the galvanometers 41 and 51 are sub stantially pure functions of the varying electrical characteristics of the formation traversed -by the electrodes because the direct current resistance of the cable I remains constant irrespective of the amount of cable wound upon the winch 9. However, if the usual type of cable is employed in a system energized with alternating current, the curves 60 and BI are no longer pure functions of the electrical characteristics of the formations traversed by the electrodes because the electrical characteristics of the conductors in an ordinarycable change with changes in the configuration of the cable. Thus a given length of two conductor cable in which the twoconductors are merely separated from each other .by insulating material will ordinarily have very perceptibly different impedance and coupling when the cable is stretched out straight than when a substantialportion of it is wound in a coil on a winch. In general, as such a cable is wound in a coil on a winch both the separate impedance of the twov conductors. increase (due chiefly to an increase in inductance) and the coupling between the conductors increases due .to the increase in the common magnetic fields of theconductors.

In a 'system'of the type disclosed in the Huber patent; when alternating current is used instead of direct current the. general effect of the changing impedance andcoupling characteristics of the cable as the-latter is wound and unwound from the winch is'to cause the curve or curves on the film to gradually shift their mean positions so that with a film strip of standard width I other portions of-the circuit.

To. compensate for the varying impedance "of the curves would run oif the edge of the strip during a runin a well of any appreciable depth;

- In. accordance with the circuit of Fig. 1, the

gradual drift of the curves Bil and 6|, due to the varying impedance of the cable I, is prevented by introducing compensating impedance changes in the input.conductor 6, theinductance element 30 is connected in series-therewithand the induct- ,ance of this element-is varied in proportion to the winding and unwinding of the cable on the [winch by providing the inductance coil 30 with a movable magnetic core 30a adapted to be move inwardly and outwardlyof the coil 30 by th screw 25 which, as previously mentioned, is gradually moved longitudinally inresponse to rotation .of the nut.2i coupled by the gears 22 and 23 to the shaft." and the shaft 11. By properly proportioning the dimensions of the coil 30 and .the movement of the core 30a relative to the movementof the cable, the impedance of the coil 30- can' be made to vary in inverse ratio to the -'variations in the impedance of the'conductor 6 due to the windingandunwinding of the cable I on the winch 9, thereby maintaining the impedance of the circuit between the electrode 8 and the bridge point 3| constant at all times.,- This eliminates a'ny tendency to drift of the curve 6|) due to variations in the impedance of the cable. 1

' As previously indicated the. potential applied to the galvanometer 51 should be a function only of the'potentials on the electrode I resulting from theelectric field produced by the current flowing from electrode 8., Howevenwhen the system is energized with altemating current and a cable of conventional type is employed potential will also be induced in the cable conductor 5 directly from the cable conductor 5, which potential will be superimposedn the potential picked up' by the electrode 1. If this induced potential were coni stant, it would not interfere, but,'.unfortunately,

the cable characteristics, I provide a potentiometer 62. havingoutput leads 63 and 63" connected across the; resistance 59 to impress an auxiliary I potential on the galvanometer 5.1. The potentiometer 62may comprise a slide resistor 84 con-.

nected across a battery .65 with one end of resistance 64 connected to the conductor 68 and -avariable sliding contact 66 connected tothe conductor 53'.

v This movable contact 56 is me-. chanically connected to the screw 25 so that it varies the potential impressed across resistance 59 in response to movement of the cable.. By

suitably dimensioning and proportioning the potentiometer 62 a voltage may be impressed across the resistor 59 which will exactly compensate for the-variations in potential due to characteristics of the cable I.

' The transformer 43; :coupling the output of bridge 32- to therectifier 45 functions as a filtering, element to prevent extraneous direct current potentials that may be 'presentin. the formation I therefore providethetran'sformer as a boost;

the varying and be picked up by the electrode a mim reach ing the galvanometer 41 while permitting the transmission of the alternating current of the frequency of the source 40.

The resistor 46, connected in series with the galvanometer l1 constitutes a sensitivity control whereby the maximum throw of the galvanometer may be brought within desired limits.

By using the rectifiers l5 and 56, respectively, it is possible to use direct current galvanometers 41 and 51, respectively, which in general have more desirable characteristics than do altemating current instruments. However, it is to be understood that if desired alternating current instrumen-ts maybe substituted for the rectifier-s and direct current galvanometers shown in Fig. 1.

The potentials in the output circuit of the' bridge 32 have substantial magnitudesuflicient to properly actuate the galvanometer 41. However the potentials picked up by the electrode 1 are some times relatively feeble so that even after being amplified in the amplifier 53 they are too weak to pass the rectifier .56 and properly energize the galvanometer 51. In this connection it should be notedthat many rectifiers that are otherwise well fitted for use in my system have even after amplification in the amplifier 53, may v not be above the threshold-value of the rectifier.

er to raise the level of the potentials picked up by electrode'1. To this endthe transformer Si is provided with a primary winding 61 connected through an adjusting potentiometer 68 to the alternating current source 40. The potential applied by the booster transformer 5| to the amany potential from the electrode Timerely biases the galvanometer 51 to a fixed position relative to the film 49; However, it raises the value of -pl'ifier input is constant and in the absence of allthe potentials applied to the rectifier 56 above the threshold value of the latter so that'the varying impulses picked up by the electrode 1 aremade effective on the galvanometer 51. It might be'supposed that by suitably adjusting the amplification of the amplifier 53 the booster transformer 5| could be dispensed with. There is adistinction, however. betweenthe .effects of the amplifier and the boostertransformer. The amplifier increases the potentials picked up byv electrode 1' geometrically; that is, all the potentials picked up by the electrode are multiplied bya constant which is the amplification constant of the amplifier. This causes. thegalvanometer 51 to move through a wide range in response to slight variations in the average potential picked up by electrode 1 and may cause the beam to swing clearoif the film Somecompensation for such excessive swinging 'may: be made by varying the variableresistor 58 in series with the galvanometer, but I find it 'den such increase in v rage load with: f

crease in the agnitude of the swings It is to be understood that amplifier 53 is not always necessary and in some cases may be eliminated, under which conditions the use of the booster transformer may be absolutely necessary to increase the magnitude of the impulses above the threshold value of the rectifier 56.

It might be supposed that the need of the booster transformer, or an amplifier, exists only because of the threshold characteristic of the rectifier 56 and that if the rectifier were eliminated and the impulses applied directly to an alternating current galvanometer there would be no occasion for raising the level of the impulses. However it should be noted that in most alternating, current meters the throw of the movable element is a function of the square of the electrical impulses appliedthereto and that for impulses below definite magnitudes the instruments are very insensitive. Hence it may be said that mean alternating current instruments have a threshold characteristic and the need for a booster transformer and/or an amplifier may still exist when such instruments are employed.

As illustrated in Fig. 1, no direct current filtering element, corresponding to the tansformer 43, has been shown in the secondary circuit. However it is desirable to have such a device ahead of the rectifier 56 and if the amplifier 53 is of conventional construction it will incorporate either transformer or capacity coupling elements which would effectively pass only alternating current.

In addition to the adjusting elements'previously described, I have shown in Fig. 1 a variable resistance 88 and battery adapted to be connected in series across the output of the rectifier 58. The battery 10 is adapted to be connected into the circuit by a reversing switch ll whereby its polarity may be reversed. The battery 10 produces a biasing effect, on the galvanometer 51, which effect 'may be reversed by throwing the switch H and may be varied by adjusting the operable resistor 89. This feature has been found convenient in practice-for adjusting the ean position of the curve 8| to a desired point 0 he film at the beginning of a run.

It is to be understood that during an actual run on a well no adjustments are made in the circuit except those automatically produced by the potentiometer 62 and the variable reactance 30 actuated in response to the movement of the cable.

With a cable of the type previously mentioned, namely, one containing two conductors, insulated from each other but not shielded from each other, it is quite essential that some compensating devices, such as those discussed with reference to Fig. 1, be employed because of the relatively great magnitude of the changes inthe impedance characteristics of the cable as it is wound and unwound from the winch. However I have discovered that by employing special cable structures and electrode arrangements, the variations in the electrical characteristics of the circuit due to .winding and unwinding of the cable, may be greatly reduced and in some instances reduced sufilciently to avoid the necessity of mechanical compensationas provided'ior in. Fig. 1. Even where such special cable structures and electrode arrangements do not entirely eliminate the necesments in Fig. 1 bear the same reference numerals distinguished by different letter sufilxes. Furthermore, to simplify the diagrams, most of the circuit details 01' Fig. 1 have-been eliminated in Figs. 2 and 4; and all of them have been eliminated in Fig. 6. It is to be understood that the complete circuit shown in Fig. 1 may be employed with the arrangementsshown in Figs. 2, 4 and 6, but that the impedance variations in the cables in Figs. 2, and 6, due'to winding and unwinding of the cables, will in general be much less than with the many conditions of operation the necessity of the variable compensating reactance '38 and potentiometer 82 ct Fig. 1 may be entirely eliminated when using the cable structures and connection shown in Figs. 2, 4 and 6.

Referring first to Figs. 2 and 3, the cable la therein disclosed differs from the cable I in Fig. 1 in that instead of comprising merely a pair of insulated conductors surrounded by insulating material, it comprises four separate conductors 80, 8|, 82 and 83, each individually enclosed in a mass of flexible insulating material 84, and a layer of armor wires 85 surrounding each insulated conductor and substantially indi vidually enclosing each conductor and shielding it from the other conductors. The four-complete insulated and shielded conductors are wound together into a cable and to improve the mechanical characteristics of thecable the four armored conductors may be wound about a cen-- tral core 88 05 rope.

Referring now particularly to Fig. 2? two of the insulated cable conductors, say conductors 88 and 8|, correspond to conductors Band 5, respectively, oi' the cable in Fig. 1 and are connected at their lower ends to electrodes 8a and la, respectively, and at their upper'ends to the diagonal 3|a of the bridge 32a and to the input terminal 52a of amplifier 53a respectively. The circuit of Fig. 2 differs essentially from thatof Fig. 1 in that the diagonal point 38a of bridge 3241, instead of being connected to ground at the surface, is connected to the upper end of one of the other cable conductors 82 and the conductor 54a of amplifier 53a instead of being connected to ground at the surface is connected to the upper end or the other cable conductor 83, The lower ends of these cable conductors 82 and 83 are connected to the armor 85 01' the cable which being in contact with the fluid in a drill ,hole, functions as a common ground return element'tor the system. I

By virtue of the fact that the currents fiow ing in conductors 88 and 82 are always substantially equal and opposite in direction, the currents in the two conductors tend to induce equal and opposite currents in the conductors 8| and 83 so that whether the cable is coiled on the winch or extended in the hole there is little coupling between the primary andsecondary circuits of the system within the cable. Furthermore, the fact that equal and opposite currents flow in conductors 88 and 82 and in conductors 8| and 88, the primary and secondary circuits considered independently or each other, have relatively low self-inductance (constituting in eflect non-inductive windings), thereby turple cable shown in Fig. 1, and therefore the problem of compensating for the changes in the cables will be greatly simplified. Under ther reducing any total change in inductance and impedance that. may occur due'to winding and unwinding of the cable.

' winding: of the cable.

aaaacos completely and individually enclosing-each conductor, actas effective shielding means between the. conductors to further reduce both coupling between the conductors and reduce the variations in the inductance that may occur in each individual conductor because of coiling and uncoiling of. the cable on the winch. 1

The circuit and cable construction shown in Figs. 2 and 3 hasproven very successful in practice and in many cases eliminates any necessity of compensating .at'the surfaces forchanges in electrical characteristics due to windingand unhowever, that althoughfor best results it is desirable to use both the ground return circuit in the cable (thereby necessitating the 4-conductor, cable) and the individual shielding about the conductors, that some improvement may be obtained by using either feature alone. ,Thus the operation of the circuit of Fig. 1 maybe improved by individually shielding the two conductors 5 and 6 of the cable therein disclosed, 'without changing the separate, ground return circuits.

Likewise the operation maybe improved by emplaying a 4-conductor cable, two conductors of ducting elements 90. 9| and 92 (Fig. 5) imbedded in a-mass of insulating material 93, such as rubber,'and enclosed oyer a portion of its length adjacentthe lower end in ametallic armor 9.4

which may consist of abasket weave metal fabric or.of-a layer of metal wires. This armor 94 functions as a -groundelectrode in the hole and preferably is of a'substantial length extending'upwardlyfroma point adjacent the lower end of the cable but may or may not extend over the entire length of thecable. In the circuit of Fig. 4 the armor 94 is connected to-the lower end of one of the conductors 90 and the upper end of thisconductor 90 is connected to the diagonalpoint 98b of the bridge 32b. The other conductors 9| and 92 correspond to the conductors Band 6 in' Fig l, conductor 9| being connected at its upper end to diagonal 3lb' of the bridge and at Q its lower end to the electrode 9b and the conductor 92' beingconnectedat its upper end to the input terminal 52b: :of amplifier 53b and being con-- .nected at its lower end to .the electrode lb. The other-input terminal 54b of tlfe amplifier is rounded at the surface through a conductor 55b, the same as in Fig. 1.

Itwill be observed that the arrangement shown The arrangement of Fig. 4 and Fig. 5 has, the

advantages discussed in connection with Figs. '2 and 3 that the currents-in the conductors 90 and 91 being edual and flowing in Opposite directions at any, instant, thereby greatly reduce the inductance of the circuit'comprising' these two conductors and also reduce the couplingeflect between this circuit and the conductor. 92. vThe It is to be understood,

advantage .of this arrangement is that it necessitates only a 3-conductor cable and it possesses some, but not all, of the advantages of the preferred circuit shown in Fig. 2.

An alternative cable arrangement is shown in Fig. 6 for a cable comprising four conductors-but having no metallic shielding which could be used as a ground element. In Fig. 6 the surface circuit would be identical with that shown in Fig. 2 and the four cableconductors are identifled by reference numerals corresponding to those employed in Fig. 2 but with the suffix c. In fact the only difference in arrangement shown in Fig. I

6 and that shown in Fig. 2, is that the cable I has no armoron it and the lower ends of the conductors 92c and 830 are connected to separate electrodes 95 and 96 which are in spaced relation with respect to the remaining electrodes 80 and 1cwhich are connected to the conductors 80c and tile. The electrodes 95 and 96 may be spaced longitudinally from each other as shown in Fig. 6 or they maybe coextensive longitudinally, or may even be combined into a single electrode. It is important, however, that the electrodes 95 and 96 be relatively long as-compared to the vertical thickness of the strata in the formation that are to be detected and that-the other electrodes 1c and 80 be relatively short as compared withthe thickness of the strata to be explored. If the electrodes 10 and tic-and these remarks apply to the electrodes 1 and 8 in all of the figures-' were of vertical dimension comparable to the thickness of the strata to be detected there would be no short demarcation in the curves 60 and GI (Fig. 1) as the electrodes passed from one strata to another. [0n the other hand, if the auxiliary electrodes 95 and/or 96 were also short relative to the thickness of the strata, then double read-* ings would be obtained in response to each strata; one reading being obtained when, for instance, the electrode 8c passed a strata junction and a similar reading being obtained again when the cooperating electrode 95 passed the junction. Of course where the ground connection or con-.-

. nection's are made at the surface, as shown in Fig. 1, or on the armor of the cable, as shown in Fig. 2, any possibility of double readings is eliminated; in Fig. 1 because of the fact that the ground electrodes do not move, and in Fig, 2 because of the fact that the armor extends substantially the full length of the hole from the, electrodes up to the top. a In Fig. 4, due to the fact that the one side of thesecondary circuit (the circuit associated with the galvanometer 510) is grounded at the surface, the cable conductor 92 will have appreciable self-inductance, which self-inductance will vary with the winding and unwinding of the cable. However, a change in the inductance of the secondary circuit is not so important as a change-of inductance inthe primary circuit because the currents in the latter are relatively large whereas the current in the conductor 92 will at all times be extremely minute. Therefore the potential drop in conductor 92 will under all conditions be small and under many conditions will not be suflicient to necessitatecompensation at the surface." It is to be understood, of course, that the electrodestructure at the bottom of the cable shown in Fig. 6 may, if desired, be' employed in the circuit of Fig. 2 instead of utilizing the armor Fig. 4, instead of employing a length of armor 94 On the cable .as a ground forv the conductor 95,

of the cable as a common ground. likewise. in a an auxiliary relatively long electrode, such as the electrode 05 of Fig. 6, may be employed.

In the circuits hereinbefore described the elec trodes in the hole have merely been indicated schematically. Various specific forms of electrode structures may be employed but. a preferred form of electrode structure that may be employed with the circuit of Fig. 1 is shown in Figs. '7, 8 and 9, and a preferred form of electrode structure (insofar as it differs from Figs. 7, 8 and 9) for use in the cable of Figs. 2 and 3, is shown in Fig. 10.

Referring first to Figs. '7, 8 and 9, the, electrode structure therein disclosed comprises an upper tubular member of metal I00 having a thick wall portion IOI at its upper end defining an opening for receiving the lower end of a cable and also defining an inner annular shoulder I02, and having at its lower end an outwardly extending flange I03 defining an external shoulder I04. The lower face of the flange portion I03 abuts against a block I05 of relatively strong insulating material,- such as Bakelite, and is maintained in engagement therewith by a metal ferrule I 06 threadedly secured to the block I05 by threads I01, the ferrule I 06 having a shoulder I08 bearin against the shoulder I04 on the member I00 and thereby Securing it against the block I05.

The block I05 contains a pair of metal conducting elements I09 and H0 imbedded therein, which elements are open'at their upper ends to define sockets for receiving and making electrical contact with a pair of plug elements III which are imbedded in a block II 3 of insulating material positioned within the lower end of the tubular member I00. The upper end of each of the plug elements III is recessed to define a socket of frusto-conical shape. Positioned above the block of insulating material H3 is another block of insulating material II4 having openings extending downwardly from the upper end for receiving a pair of insulated conductors H5 and H6 of a 2-conductor cable III which projects into the tubular member IOI, first mentioned. The passages through the block II 4, for passing the insulated cable elements H5 and I I6, are enlarged at their lower ends to receive metal wedge elements II8 which have plug ends of frusto-conical shape adapted to nest into the sockets in the upper ends of the plug members III and having relatively small holes extending centrally therethrough for receiving the metal conducting elements I 20 of the insulated elements H5 and H6. A .block I2I of relatively strong material (preferably metal) is positioned within the tubular member IOI above the block II4, this block I 2| having a pair of diverging holes for passing the elements H5 and Hi, the lower ends of these openings being in registration with the upper ends of the openings in block I I4. The upper end of block I2I is pressed against a gasket I22 bearing against the shoulder I02 in the member I00. Gasket material I23 ,is also inserted between the blocks H4 and I2I, and gasket material I24 is inserted between the blocks H4 and II3. All three blocks I2I, H4 and H3 are secured tightly together by a pair of bolts I25 forces the lower plug ends of the members H8 tightly against the upper socket ends of the members III to tightly grip the spread wires 01 the cable conductors I20 therebetween. It is to be understood that the cable conductors I20 not only serve as electrical conducting elements but also as the tensile supporting elements in the particular cable disclosed, and it is therefore necessary to anchor the ends of these conductors for mechanical reasons as well as to effect electrical conduction.

The upper portion of the electrode structure so far described is assembled as follows:

First a length of rubber hose I2! is slipped over the lower end of the cable and the hose and cable are inserted downwardly through the opening in the tubular member IOI for a substantial distance therethrough and the lower end of the cable is prepared by speading the elements H5 and IIS and stripping away the ends of the insulating material on those elements to uncover the conducting elements I20. At this time the ferrule member I00 is mounted on the tubular member I00 but is slid upwardly therealong'out of the way. The gasket I22 is next placed over the end of the cable and the ends of the members H5 and IIS passed through the openings in the block I2I. Then the gasket I23 and the block II4 are slipped over the members so that the ends of the conductors I20 project through theopenmgs in the members H8 and a distance therebeyond; These conductors are then spread to bend back along the frusto-conical surfaces of the plug members H8 and the block H3 is placed in position with the upper socket ends of the plug members III therein against the spread conductors on the plug members 8. Next the bolts I25 are inserted and screwed up tight, thereby compressing the blocks I2I,I I4 and H3 tightly together against the gaskets intervening between them and the compression between blocks H3 and H4 wedges the spread strands of conductors I20 tightly between the members H8 and the members I I I thereby providing mechanical anchorage between the cable elements I20 and the assembly of blocks, and also establishing electrical contact between the conductors and the plugs III.

' The cable I" is then withdrawn upwardly through the tubular member IOI carrying the blocks I2I, H4 and 3 up into the sleeve I00 after which the members I 05 and I03 are slipped together with he plugs III engaging the socket ends of the conductor members I09 and H0 and the block I05 secured firmly against the lower end of the tubular member I03 by slipping the ferrule I06 down over the member IN and screwing it tightly onto the member I05.

g The lower portion of the electrode structure comprises a steel tube I30, the upper end of which slips over the lower portion of member I05 and is secured in place by means of a plug I 3I which is connected to the member I05 by a stud I32, the plug I3I being attached to the sleeve I30 by a pin I33 extending therethrough. The sleeve I30-constitutes a part of one electrode connected. to the connecting member IIO. To this end the tube I30 is connected to the member by a screw I34.

Referring now. to Fig. 9 the tube I30 extends downwardly below the block I05 for a substantial distance and at its lower end is attached to a connector member I 35 by a pin I38. The connector member 135 has a central passage I31 extending therethrough and the pin I38 is preferably offset from the center so as not to obstruct the passage I31. The lower portion of the exterior surface of the connector member I35 is aazaeoa of slightly larger diameter than the upper end so that it is flush with the exterior surface of the tubing I30. The connector member I35 is the interior of the tubing I40.

It is usually desirable to an the Sp... within the tubing sections I30 and I40 with some heavy material to impart weight to the structure. It

.has been found convenient to fill the upper tubing section I30 with lead balls small enough to pass through the passages I31, these balls being,

inserted 'after assembly of the connector member I38 to the lower end of the tubing I30 but before the lower section including the connecting member I38 is screwed into place. The lead balls may be of such diameter that they will pass through the passage I3I but will not pass through the passage MI. The space within the tubing sectionv I40 may be filled with lead discs inserted prior to the attachment of the end piece I43. These discs preferably have central apertures therein to permit the flow of fluid through the interior of the device. I v I As previously stated, the tubing I30 and also the tubing I40 which is connected to tubing I30 .by connectors I35 and I38 constitute a part of one electrode. The end piece I43 constitutes the remairing portion of this electrode. It has also been s ated previously that it is desirable that the electrodes in an electrical surveying. system of the type disclosed be of limited eflective vertical length. To this end the end piece I43 is the only portion of the electrode structure so far described that'is exposed on its exterior to the well fluid. The exposed portion of this end piece I43 is of relatively limited length. However, it

should be observedthat well fluid can penetrate through the passage I44 freely into the interior of the tubes I40 and I30,'thereby contacting a *large surface inside the electrode structure.

Howevercurrent can only reach or leave the electrode either from the outer surface of the end piece I43 or through the liquid in the passage I 44 from the interior-of the electrode. The structure described therefore provides a large contact area with well fluid and yet confines the current leaving the exterior of the electrode to a very small area'measifi'ed vertically.

To prevent electrical contact between well fluid and .the exterior of the tubing sections I30 and I40, each is surrounded with a layer of rubber.

. Thus the tubing section I30 is enclosed in a layer of rubber I45 which extends clear from the upper end of the tubing section to the lower end of the connector element I35 completely enclosing the latter. The lower tubing section I40 likewise is covered with a layer of rubber I46 the upper end of which abuts against the end of the lower end of the rubber layer I45 and seals there- Y with and the lower end extends to the lower end of tubing I40 and abuts against a shoulder the metal ferrule member I05, the tubular memstructure.

ber WI, and up along the outside of the rubber hose I2'I which encloses thecable for a distance thereabove. I

The second electrode element which is also 'of limited vertical extent and isinsulated from.

and spaced vertically from the exposed area of the electrode structure described,v comprises a thin metal band I50 mounted in a. groove or circumferential recess provided therefor in the outer surface of the rubber layer I45. The outer surface of the band I 50 is preferably in-set or countersunk with respect to the outer surface of the rubber I48 so that the possibility of the band" directly contacting the side wall of the hole is diminished. The bandI50- is connected to the conductor member I09 by a thin metal strip I5I which is connected at its lower end to the band I50 and extends upwardly therefrom through the rubber wall I45 to a point adjacent the upper end thereof, where it is connected'by a screw I52 to the conductor member I09 The tubing I30" is provided with a recess I53 to permit passage through the tubing of the screw I52 without electrical contact therebetween. The strip I5I is completely imbedded in the rubber wall I45 and thereby insulated from both the tube I and from well fluid'exterior of the rubber.- As shown in the drawings, holes are provided in the rubber wall I opposite the screws I 34 and I52,- and have been inserted these holes are filled.with

outer surface water-tight. I

The electrode structure described. with reference to Figs. '7, 8 and 9, involves only two electrodes. It will be obvious, however, that any desired number of bands similar to band I may be positioned at longitudinally spaced points along the structure and each connected to a" thin metal strip imbedded in the rubber extending thereabove for completing connection to a cable conductor. Instead of bands, segments may be used, which extend only partway around-the It will be observed that the joint members.l35 and I38 efiect-electrical connection to the lower electrode element I43 so thatlt is a simple operation to change the lower'end section. This is advantageousfas it is sometimes desirable to vary the spacing between electrode I43 and electrode-I50. With the construction ferent length. v

In Fig. 8 the cable III is shown covered with a basket weave armor but it is not essential that any armor be employed if the arrangement is used in a circuit such asthat shown in Fig. 1. However if it is desired to employ the'structure of Figs. 8 and 9 witha 3-co'nductor cable such as illustrated in Fig. 4, then one of the.condu c.tors

ually armored-conductors II8d pass through a block-l2ld'and through passages in a block I55,

' thepass'ages tapereing from a small diameter at the-top to a large diameter at the bottom. The

armor is spread away from the insulated memwould be connected to the armor of the cable where the latter e'ntersthe electrode structure.

pins I38, I38 and I42 After the screws and pins v 50 shown, such variation in the spacing may be made by substituting a lower end section of difher therebelowso that it lies out against the surface of the conical passage and a hollow-conical block I56 is positioned above the insulated conductor inside of the armor wires so as to wedge the latter against the walls of the recess and thereby mechanically anchor the cable armor to the block I55. It should be observed that in the type of cable disclosed in Fig. 10 and also in Fig. 3, the armorwires constitute the tensile :supporting elements of the cable so that the electrical conductors do'not take the strain.

The insulated electrical conductors within the armor extend down through the central openings in the wedge members I and through registering openings in a block of insulating material I51 therebelow. At the lower end of the block I5! the openings are enlarged and receive metal sockets I58 to which the electrical conductors are soldered or otherwise secured. These sockets I58 are adapted to be engaged by plug ends I 59 of metal conducting elements I09d and' I Illd positioned in a block of Bakelite, or other insulating; material- 15d, which corresponds in function to the members I09, III! and I05 in Fig. 8.

Of course in the construction shown in Fig. 10,

there are as many wedge elements I56 as there are separate armored conductors in the cable. However, when the structure is employed in a system of the type shown in Fig; 2, only two of the cable conductors are electrically connected to the electrode structure and these may be connected to two electrodes as described in connection with Figs. 8 and 9. 'The remaining two conductors are electrically connected to the cable armor and to this end this connection may be made in any convenient manner. An obvious way to make the connection is to remove the insulation from the central conductors at the upper ends of the plugs I 56 and extend the conductor wires out alongside the armor wires and clamp them with the armor wires between the .plug

members I56 and the walls of the sockets in the block I55. That portion of the armor structure which contacts thewell fluid may be spaced a desired distance above the electrodes on the lower end of the electrode structure described by extending the hose IZ'Id and/or the outer rubber ferrule Il'ld the desired distance up along the exterior of the cable. a

Although certain specific embodiments of the invention have been described in detail fonthe purpose of clearly illustrating and explaining the invention, it is to be understood that many varia- I tions from the specific constructions and arrangements described can be made without departing from the invention, and the latter is tobe limited only to the extent set forthin the appended claims.

I claim:

1. Apparatus for electrical earth exploration in drill holes comprising two spaced apart electrodes adapted to be positioned in a drill hole,

' a cable comprising a pair of insulated conductors connected at their lower ends to said respective electrodes, means for winding up said'cable to connected in circuit with said indicating means for addinga potential to said indicated potentials to compensate for-the effects on said indicating means resulting from changes in the electrical characteristics of said cable produced by windrent through one of said conductors and from the electrode connected thereto into the surrounding formation, means connected to the other electrode through the conductor connected thereto for measuring potential variations in the other electrode as the cable is moved through the drill hole, andimeans for superimposing on the potential applied from said other electrode to said measuring means, a substantially constant potential to increase the potential supplied to said measuring means to a predetermined average level higher than the average level of the potential variations of said other electrode.

3. Apparatus for exploring the formation traversed by a drill hole containing conducting fluid, comprising a cable containing a pair of insulated conductors terminating in a pair of spaced apart electrodes, means for moving said'- cable through the drill hole, means for producing an alternating current through one of said conductors and from the electrode connected thereto into the surrounding formation, means connected'to the other electrode through the conduetor connected thereto for measuring potential variations in the other electrode asthe cable is moved through the drill hole, means for superimposing on the potential applied from said other electrode to said measuring means a substantially constant alternating potential of the same frequency as said source but of reduced average value to increase the potential supplied to said measuring means to a predetermined average level higher than the average level of the potential variations of said other electrode.

4. Apparatus for exploring .the formation traversed by a drill hole containing conducting fluid, comprising a cable containing a pair of insulated conductors terminating in a pair of spaced apart electrodes, means for moving said cable through the drill hole-means for produc-- ing an alternating current through one of said conductors and from the electrode connected thereto into the surrounding formation, means connected to the other electrode through the conductor connected thereto for measuring'potential variations in the other electrode as the cable is moved through the drill hole, and means for deriving fromsaid source a potential 'of the same frequency but of reduced average value and superimposing it upon the potential applied from said other electrode to said measuring means, whereby the potential applied from said other electrode to said measuring means isincreased to a predetermined average level.

5. Apparatus for exploring the formation traversed by a drill hole containing conducting fluid comprising a cable containing a pair of insulated conductors terminating in a pair of spaced apart electrodes, means for moving said cable through the drill hole, means for producing ,a flow of alternat ng current through one of said conductors an from the electrode connected thereto into the surrounding formation, means connected to the other'electrode through the conductor conof alternating potential, and means for applying trode vertically spaced from said long electrode nected thereto for measuring potential variations in the other electrode as said cable is moved through the drill hole, said measuring means comprising a rectifier having an input circuit:

connected to said other electrode and having an output circuit, a direct current indicating instrument connected to the output circuit of said rectifier, and a source of direct current of ad- Justable resistance connected in shunt to the outputcircuit of said rectifier and instrument.

6. Apparatus for exploring the formation traver'sed by a drill hole containing conductive fluid comprising a cable having four insulated conductors adapted to be lowered into the drill hole from/the surface, means at the surface for coiling and uncoiling said cable to raise and lower the lower end of the cable'in the hole, a source potential therefrom between the'upper ends of a first pair of said insulated'conductors, a long electrode exposed to the'con'ducting fluid in said drill. hole and connected to the lower end of one of the conductors of, said first pair, a short electrode' vertically spaced from said long electrode and connected to the lower end of the other conductor of said first pair, means for indicating potential variations connected between the upper ends of a second pair of said insulated conductors, a long electrode connected to'the lower end of one conductor of said second painand a short electrode vertically spaced from all the previously mentioned electrodes connected to the lower end ofthe other conductor of aid second pair. 1 7. Apparatus as described in claim 6, in which said cable comprises conducting means insulated om but individually shielding each of said c nductors from the other conductors of the cable.

8. Apparatus as described in claim 6,-including means for indicating impedance variations in the circuit including said first pair of said conductors and the electrodes connected thereto.

. 9. Apparatus as described in claini 6, in which the vertical spacing between the two short glectrodes is substantially less than the vertical spacdrilLhole and-connected to the lower end of one of theconductors of'said flrst pair, a short elecand connected to the lower end of' the other conductor of said first-pair, meansfor indicating wpotential variations connectedbetween the upper ends of a second pair ofinsulated conductors, one conductor of said second pair being connected at its lower end to said long electrode, and,.a

short electrode verticallysp aced from 'all the previously mentioned electrodes connected to the between the upper ends of a said armor constitutes one at least of said long electrodes.

12. Apparatus for exploring the formation traversed by a drill hole containing conductive fluid, including a cable-comprising two insulated conductors, and an exterior conductive armor, means at the. surface for coiling and uncoiling the cable to raise and lower the end of the cable in the hole, a source of potential and means for applying potential therefrom between the upper ends of said insulated conductors, the lower end of one of ghe conductors being connected to said armor at the lower end of said cable, a short electrode connected to thelower end of the other conductor, and electrical measuring means connected to the upper ends of said-conductors, in which said armor extends substantially to the said electrode on the lower end of said cable, and means for insulating said armor from fluid in said drill hole for a predetermined distance extending upwardly from the said electrode.

13. Apparatus for exploring the formation traversed by a drill hole containing conductive fluid comprising a cable having three insulated conductors adapted to be lowered into the drill hole from the surface, two electrodes on said cable connected'to the lower ends of two of said conductors, a return circuit to ground for each electrode, the ground return path for one at least of said two circuits comprising the third conductor in said cable, and a third electrode connected to the lower end of said third conductor, said third electrode being in spaced'relation to said first two electrodes, and said third electrode having contact surface exposed to the fluid in the drill hole over a substantially greater length than said first and second electrodes.

14. In apparatus for determining varying alternating current characteristics of earth formations penetrated by a drill hole containing conductive fluid, in which a cable having electrically insulated conductors connected at "their lower end to electrodeaand connected at their upper ends to measuring apparatus, is coiled and uncoiled to .va'ry the positions-of the electrode, the cable construction comprising, in addition to said electrically insulated conductors. other conducting means insulated from and individually enclosing each of said conductors, whereby variations in the impedance of said conductors and variationsin the coupling between said conductors resulting from coiling and un'coiling from saidcable, are reduced.

' fluid, a cable having a tensile supporting element,

an insulated electrical conductor element, and a terminal electrode. structure comprising an upper frame member and a lower frame member,

and means for detachablyfastening them-together, said members having abutting and seal- 1 ing cooperating end faces,means for anchoring said tensile supporting element ofsaid cable in said "upper, frame member, a slip connector in said upper frame member, and means for conducting said electrical conductor element of said cable thereto, a slip connector in said lower frame member adapted to connect withithe'slip connector in the upper frame member in response to joinder 'of said twoframe members, and an exposed electrode elementon said lower frame member electrically connected to the slip con-' nector on the lower frame member,

"16. Apparatus as described in claim 15, in'

which the means for anchoring said tensile element of the cable comprises a socket member having an inner surface and cooperating plug members having a conical outer surface, and means for clamping said socket member and plug member together to grip said tensile member of the cable therebetween.

17. Apparatus as described in claim 15, in which the cable comprises an insulated con: ductor surrounded with a layer of armor wires constituting the tensile element, and said means for anchoring said tensile element comprises a socket member having a conical inner surface and a cooperating plug member having a conical outer surface, and means for clamping said socket member andplug member together to grip said armor wires therebetween, said socket and plug members having central openings extending therethrough for passing said insulated conductor.

18. An electrode structure adapted to be suspended on a cable containing an insulated conductor for taking electrical measurements in drlllg holes, comprising a rigid frame member, means for anchoring it to a cable, a layer of resilient insulated material extending circumferentially around said frame member and longitudinally therealong for asubstantial distance and an electrode element comprising a metallic insert imbedded in said resilient material with its outer surface exposed, and means for connecting said elegltrode element to the insulated element of the 19. Apparatus as described in claim 18,- including connector means on said frame and insulated therefrom adjacent one end of said structure adapted to be connected to a cable conductor, said connector means being spaced longitudinally from said metallic insert, and means electrically connecting said insert to said connector means comprising a thin strip of metal completely imbedded in said insulating material.

20. In electrical exploration of well holes containing conductive fluid, an electrode for electrically contacting fluid at two longitudinally spaced surfaces of limited effective length, one of which electrodes has relatively low total $111:- l'ace resistance with the fluid, comprising an inner hollow conductive member of substantial length sealed at one end andopen at the other end to well fluid, said member constituting one of said electrodes, a sheath of insulating material surrounding said surface exterior of said hollow member at all points except closely adjacent the open end thereof, and' a second electrode rigidly supported on said insulating sheath in spaced position from said open end of said member.

21. In apparatus for making electrical measurements in bore holes comprising a cable containing a plurality of insulated conductors, electrode means on the lower end of the cable and connected to said conductors, electrical measuring means connected throughv said insulated conductors to said electrodes, and means for coiling and uncoiling the cable at the surface to raise and lower the electrodes in the hole; the cable construction comprising: a plurality-of separate strand members twisted together, eachstrand member comprising a central conductor surrounded with flexible insulating-material and a layer of armor wires surrounding and enclosing said insulating material.

22. Apparatus for electrical exploration in drill holes comprising a cable having at least two conductors, means for coiling and uncoiling saidcable to move it in a bore hole, means inaaaaeoa cluding one of said conductors for passing cur-' rent through: the formationsurrounding the bore hole, a variable impedance element in the circuit of said conductor for compensating for the changing conductor impedance resulting from coiling and uncoiling said conductor and means including a'se'cond conductor for obtaining indications of potentials produced in the bore hole by current passing through the formations.

23. Apparatus for electrical exploration in -drill holes comprising a cable having at least two conductors, means for coiling and uncoiling 'said cable to move it in a bore hole, means including one of said conductors for passing current through the formation surrounding the bore hole, a variable impedance element in the circuit of said conductor, means for controllin said impedance element to vary its impedance in a manner complementary to the variations in conductor impedance resulting from coiling and uncoiling said conductor and means including a second conductor for obtaining indications of potentials produced in the bore hole by current passing through the formations.

24. Apparatus. for electrical exploration in drill holes comprising a cable having at least two conductors, means for coiling and uncoiling said cable to move it in a bore hole, means in-. cluding one of said conductors for passing current through the formation surrounding the bore hole, a' variable impedance element in the circuit of said conductor, means responsive to movement of said cable for controlling said impedance elemeht to vary its impedance in a manner complementary to the variations in conductor impedance resulting from coiling and uncoiling said conductor and means including a second conductor for obtaining indications of potentials 25. Apparatus for electrical exploration in drill holes comprising a cable-having at least two conductors, means for coiling and uncoiling said cable to move it'in a drill hole, means including one of said conductors for establishin an alternating electric fleld in the formation surrounding the drill .hole, means including a second conductor for measuring the potentials in the drill'hole resulting from the said electric field; and means for adding a variable potential to said measured potentials to compensate for variations in potentials induced'in said second conductor by said first conductor, resulting from coiling an'd uncoiling said-cable.

26. Apparatus for electrical exploration in drill holes comprising a cable having at least two conductors, means for coiling and uncoiling said cable to move it in a drill hole, means including one of said conductors for establishing an alternating electric field in the formation surrounding the drill hole, means including a second conductor for measuring the potentials in the drill hole resulting from the said electric field, means for adding a variable potential to said measured potentials, and means for controlling said potential adding means to vary the potential added to the measuring circuit to compensate for variations in potentials induced in said second conductor by said first conductor, resulting from coiling and uncoiling said cable.

27. Apparatus for electrical explorationin drill holes comprising a cable having at least two conductors, means for coiling and uncoiling said cable to move it in a drill hole, means including one of said conductors for establishing an alterhating electric field in the formation surrounding-the drill hole, means including a second conductor for measuring the potentials in the drill hole resulting from the said electric field', means for adding a variable potential to said measured potentials, and means responsive to movement of said cable for controlling said potential adding means to vary the potential added to the measuring circuit to compensate for the effects of variations in pa ntials induced in said second conductor by said rst conductor resulting from coiling and uncoiling said cable. 7

28. Apparatus for electrical exploration in drill holes comprising a cable having at least two conductors, means for coiling and uncoiling said cable to move it in a drill hole, means in:-

eluding one ,of said conductors for establishing an alternating current field in the formation surrounding a drill hole, a variable impedance element in the circuit oi, said conductor for compensating for the changing conductor impedance resulting from coiling and uncoiling said conductor, means including a second conductor for measuring the potentials in the drill .hole result-- ing from said electric field, and means for add- 7 means.

ing a potential to said measured potentials to compensate for variable. potentials induced in said second conductor by said first conductor,

resulting from coiling and uncoiling said cable..

29., Apparatus for electrical exploration in drill holes, comprising a cable having at least two conductors, means for coiling and uncoiling 'said cable to move it in a drill hole, means including one of said conductors for establishing an alternating current field in the formation surrounding the drill hole, a variable impedance element in the circuit of said conductor for compensating for the changing conductor impedance thereof resulting from coiling and uncoiling said cable, means including a second conductor ,for

measuring potentials in the drill hole resulting from said electric field, means for adding a potential to said measured potentials to compensate for variable potentials induced in said second conductor by said first conductor, resulting from coiling and uncoiling said cable, and means responsive to winding and unwinding movement of I said cable for actuating both said variable impedance element and said potential adding RAYMOND D. Emo'rr. 

